1. Field of the Invention
The present invention relates to an electromagnetic antenna and to an excitation antenna for a nuclear magnetic resonance (NMR) apparatus equipped with an electromagnetic antenna of this type. Although a primary application is in the medical field in which NMR devices are employed for diagnosis, the invention is also useful in other fields for reducing the influence of an electric field radiated by, or received by, a radiating circuit.
2. Description of the Prior Art
An NMR imaging device is essentially equipped with means whereby a patient's body to be examined is subjected to a uniform, constant and high-strength magnetic field B.sub.O. While the body is subjected to this influence, a radiofrequency electromagnetic excitation is then applied to the body by an excitation antenna in order to induce resonance of oscillation of the magnetic moments of body particles. After this excitation, a measurement is performed on the resonance signal emitted as feedback by the body when the magnetic moments of the particles tend to realign themselves with the field B.sub.O. For certain regions of the body such as the head, for example, transmitter-receiver antennas are employed. These antennas, which are represented schematically by a radiating loop, produce and pick-up the magnetic fields. By virtue of the distribution of potential within the circuit formed by said loop, there is produced an electric field which does not contribute to formation of the image. At the time of transmission, this electric field in fact lowers the efficiency of the antenna by causing losses by current induced within the patient. Thus the entire quantity of energy is not employed for establishing the high-frequency magnetic field. On reception, the radiating loop collects from the resonance signal an electric field component which produces a disturbance in the magnetic field component and which it is sought to measure alone. An attempt has already been made to solve this problem by reducing the mean electric field produced or picked-up by the antenna. In practice, steps are taken to ensure that a midpoint of said antenna is maintained at a constant potential at the moment of resonance. Two ends of the antenna then resonate in phase opposition and produce or pick-up electric fields in phase opposition having an intensity of one-half the value of the electric fields which would be produced or picked-up by the midpoint of said antenna if its two ends were maintained at equal and constant potentials. While achieving an improvement, this solution is not sufficient for ensuring efficiency of the antenna.